Oxide magnetic material and sintered magnet

ABSTRACT

An oxide magnetic material according to the present invention is represented by the formula: (1−x)CaO.(x/2)R 2 O 3 .(n−y/2)Fe 2 O 3 .yMO, where R is at least one element selected from the group consisting of La, Nd and Pr and always includes La, M is at least one element selected from the group consisting of Co, Zn, Ni and Mn and always includes Co, and the mole fractions x, y and n satisfy 0.4≦x≦0.6, 0.2≦y≦0.35, 4≦n≦6, and 1.4≦x/y≦2.5. The oxide magnetic material includes a ferrite having a hexagonal M-type magnetoplumbite structure as a main phase.

TECHNICAL FIELD

The present invention relates to an oxide magnetic material and asintered magnet, each including a ferrite with an M type magnetoplumbitestructure as a main phase, and methods of making them.

BACKGROUND ART

Ferrite is a generic term for any compound including an oxide of adivalent cationic metal and trivalent iron, and ferrite magnets havefound a wide variety of applications in numerous types of rotatingmachines, loudspeakers, and so on. Typical materials for a ferritemagnet include Sr ferrites (SrFe₁₂O₁₉) and Ba ferrites (BaFe₁₂O₁₉)having a hexagonal magnetoplumbite structure. Each of these ferrites ismade of iron oxide and a carbonate of strontium (Sr), barium (Ba) or anyother suitable element, and can be produced at a relatively low cost bya powder metallurgical process.

Recently, it was proposed that the coercivity HcJ and the remanence Brof an Sr ferrite be increased by substituting a rare-earth element suchas La for a portion of Sr and by substituting Co for a portion of Fe(see Patent Documents Nos. 1 and 2).

Just like the Sr ferrite, it was also proposed that a rare-earth elementsuch as La be substituted for a portion of Ca in a Ca ferrite and thatCo be substituted for a portion of Fe (see Patent Document No. 3).

-   -   Patent Document No. 1: Japanese Patent Application Laid-Open        Publication No. 10-149910    -   Patent Document No. 2: Japanese Patent Application Laid-Open        Publication No. 11-154604    -   Patent Document No. 3: Japanese Patent Application Laid-Open        Publication No. 2000-223307

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

As for Ca ferrites, it is known that the structure CaO—Fe₂O₃ orCaO-2Fe₂O₃ has good stability and produces a hexagonal ferrite when Lais added thereto. However, the resultant magnetic properties are as goodas those of a conventional Ba ferrite and are not sufficiently high.Thus, to increase the remanence Br and coercivity HcJ and improve thetemperature dependence of the coercivity HcJ, Patent Document No. 3discloses a Ca ferrite including both La and Co (which will be referredto herein as a “CaLaCo ferrite”).

In the CaLaCo ferrite disclosed in Patent Document No. 3, a portion ofCa is replaced with a rare-earth element such as La and a portion of Feis replaced with Co and so on. As for its anisotropic magnetic fieldH_(A), it is reported that a value of 20 kOe or more, which is more than10% higher than the anisotropic magnetic field H_(A) of an Sr ferrite,was achieved at best.

According to the working examples of the CaLaCo ferrite disclosed inPatent Document No. 3, if x=y=0 to 1 and z=1 inCa_(1-x1)La_(x1)(Fe_(12-x1)CO_(x1))_(z)O₁₉, good properties are achievedwhen x=y=0.4 to 0.6. More specifically, Br=4.0 kG (=0.40 T) and HcJ=3.7kOe (=294 kA/m) were achieved when baking was carried out in the air andBr=4.0 kG (=0.40 T) and HcJ=4.2 kOe (=334 kA/m) were achieved whenbaking was carried out in oxygen (i.e., in 100% oxygen).

On the other hand, if the z value was decreased to 0.85 (and x=0.5,y=0.43 and x/y=1.16) in the composition described above, Br=4.4 kG(=0.44 T) and HcJ=3.9 kOe (=310 kA/m) were achieved when baking wascarried out in the air and Br=4.49 kG (=0.449 T) and HcJ=4.54 kOe (=361kA/m) were achieved when baking was carried out in oxygen (i.e., in 100%oxygen). The latter property is the best property achievable by PatentDocument No. 3.

An Sr ferrite in which a portion of Sr is replaced with a rare-earthelement such as La and a portion of Fe is replaced with Co and so onaccording to Patent Documents Nos. 1 and 2 (which will be referred toherein as an “SrLaCo ferrite”) has such excellent magnetic properties asto be often used in various applications in place of the conventional Srferrite or Ba ferrite.

Ferrite magnets are used most frequently in motors. If the magneticproperties of a ferrite magnet improve, then the output of a motor canbe increased or the size thereof can be reduced. That is why it is veryeffective to increase the remanence Br, coercivity HcJ and maximumenergy product (BH)max. However, not only these properties but also theloop squareness Hk/HcJ must be good. This is because if the loopsquareness were poor, then the critical demagnetization field strengthwould be so small as to cause demagnetization easily. In motors amongother things, when embedded in a magnetic circuit, a ferrite magneteasily demagnetizes, which is regarded as a non-negligible problem. Toovercome such a problem, there is a high demand for a high-performanceferrite magnet that has high coercivity HcJ (or high coercivity HcJ andhigh remanence Br) and good loop squareness. It should be noted that theparameter Hk, which is measured to evaluate the loop squareness, is anabscissa (i.e., an H value) corresponding to a 4 π I value of 0.95 Br inthe second quadrant of a 4 π I (degree of magnetization)-H (strength ofmagnetic field) curve. The Hk/HcJ ratio, calculated by dividing this Hkvalue by the HcJ of the demagnetization curve, is defined as the loopsquareness.

The CaLaCo ferrite according to Patent Document No. 3 has as goodmagnetic properties as the SrLaCo ferrite and is a material, of whichthe applications are expected to expand greatly in the near future. Butthe CaLaCo ferrite has very bad loop squareness Hk/HcJ. As describedabove, according to Table 2 in Example 2 of Patent Document No. 3,excellent properties including Br=4.49 kG (=0.449 T) and HcJ=4.54 kOe(=361 kA/m) are achieved but its loop squareness is only 80.6%.

In FIG. 14 of Patent Document No. 3 (see Example 10 thereof), shown isthe loop squareness in a situation where x1=0 to 1 inCa_(1-x1)La_(x1)Fe_(12-x1)Co_(x1). However, when x=y=0.4 to 0.6, whichis a preferred range according to Patent Document No. 3, the loopsquareness is about 80%. If x1 is 0.8, then the loop squareness exceeds85% but the coercivity HcJ decreases steeply.

Also, in FIG. 15 of Patent Document No. 3 (see Example 11 thereof),shown is the loop squareness in a situation where x2=0, 0.2 or 0.4 inSr_(0.4-x2)Ca_(x2)La_(0.6)Fe_(11.4)Co_(0.6). The loop squareness exceeds90% when x2=0 or 0.2 (i.e., in a range where there is a lot of Sr) butis 80% or less when x2=0.4 (i.e., full Ca). In this case, the behaviorof the coercivity HcJ is quite opposite to that of the loop squareness.And the highest coercivity HcJ is achieved when x2=0.4 (i.e., full Ca).

As described above, the CaLaCo ferrite according to Patent Document No.3 exhibits better properties than the SrLaCo ferrite as for theanisotropic magnetic field H_(A) and its Br and HcJ are as high as thoseof an SrLaCo ferrite. However, its loop squareness is poor and highcoercivity and good loop squareness cannot be satisfied at the sametime. Consequently, the CaLaCo ferrite cannot be used in motors andvarious other applications yet.

In order to overcome the problems of the conventional CaLaCo ferritedescribed above, an object of the present invention is to provide anoxide magnetic material and a sintered magnet that have increased Br andHcJ and improved loop squareness.

Means to Solve the Problems

This object is achieved by any of the following subject matters:

(1) An oxide magnetic material having a composition represented by theformula:

(1−x)CaO.(x/2)R₂O₃.(n−y/2)Fe₂O₃ .yMO,

where R is at least one element selected from the group consisting ofLa, Nd and Pr and always includes La,

M is at least one element selected from the group consisting of Co, Zn,Ni and Mn and always includes Co, and the mole fractions x, y and nsatisfy

0.4≦x≦0.6,

0.2≦y≦0.35,

4≦n≦6, and

1.4≦x/y≦2.5,

wherein the oxide magnetic material includes a ferrite having ahexagonal M-type magnetoplumbite structure as a main phase.

(2) The oxide magnetic material of (1), wherein 4.8≦n≦5.8 is satisfied.

(3) A sintered magnet comprising the oxide magnetic material of (1) or(2).

(4) The sintered magnet of (3), wherein the sintered magnet has acoercivity HcJ of 370 kA/m or more.

(5) The sintered magnet of (3), wherein the sintered magnet has aremanence Br of 0.45 T or more.

(6) The sintered magnet of (3), wherein the sintered magnet has a loopsquareness Hk/HcJ of at least 85%.

(7) The sintered magnet of (6), wherein the loop squareness Hk/HcJ is90% or more.

(8) A method of making the oxide magnetic material of (1) or (2),comprising the step of adding at most 0.2 mass % of H₃BO₃ before and/orafter the step of calcining.

(9) A method for producing the sintered magnet of (3), comprising thestep of adding 1.0 mass % or less of SiO₂ and 1.5 mass % or less ofCaCO₃, when converted into the mass of CaO, before the step of finepulverization.

(10) A method of making the oxide magnetic material of (1) or (2),comprising the step of setting the oxygen concentration of a calciningatmosphere at 5% or more.

(11) A method for producing the sintered magnet of (3), comprising thestep of setting the oxygen concentration of a sintering atmosphere at10% or more.

(12) An oxide magnetic material consisting essentially of Ca, La, Fe andCo and comprising a ferrite having a hexagonal M-type magnetoplumbitestructure as a main phase, wherein the oxide magnetic material includessubstantially no hetero phases including a lot of Co.

EFFECT OF INVENTION

According to the present invention, an oxide magnetic material that hasincreased Br, increased HcJ and improved loop squareness can beprovided.

If a sintered magnet is made of the oxide magnetic material of thepresent invention, a coercivity HcJ of 370 kA/m or more is achieved in apreferred embodiment and a remanence Br of 0.45 T or more is achieved ina preferred embodiment.

If a sintered magnet is made of the oxide magnetic material, a loopsquareness of at least 85% is achieved in a preferred embodiment, and aloop squareness of 90% or more is achieved in an even preferredembodiment.

According to the present invention, Br and HcJ that are higher thanthose of the SrLaCo ferrites disclosed in Patent Documents Nos. 1 and 2are achieved.

Also, according to the present invention, properties that are at leastcomparable to Br and HcJ in a situation where the CaLaCo ferrite ofPatent Document No. 3 is baked in oxygen (in 100% oxygen) are achieved(i.e., the best properties of Patent Document No. 3 are achieved) evenwhen the baking process is carried out in the air. This is effectivebecause the baking process in the air can be carried out more easily,and guarantees more constant production, than in oxygen.

A sintered magnet according to the present invention has such high Br,high HcJ and good loop squareness as to find its best applications inmotors and so on.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is graphs showing how the remanence Br, coercivity HcJ and Hk/HcJof a sintered magnet change with the molar ratio x/y in a situationwhere n=5.4 and the x/y ratio of the mole fraction x of substituent Lato the mole fraction y of substituent Co is changed from 1.0 to 5.0 inthe composition (1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO.

FIG. 2 is graphs showing how the remanence Br, coercivity HcJ and Hk/HcJof a sintered magnet change with the mole fraction y in a situationwhere x=0.50, n=5.4 and y is changed from 0 to 0.50 in the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO.

FIG. 3 is graphs showing how the remanence Br, coercivity HcJ and Hk/HcJof a sintered magnet change with the mole fraction n in a situationwhere x=0.50, y=0.30 and n is changed from 3.6 to 6.0 in the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO.

FIG. 4 is graphs showing how the remanence Br, coercivity HcJ and Hk/HcJof a sintered magnet change with the mole fraction x in a situationwhere 0.00≦x≦1.0, y=0.3 and n=5.2 in the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO.

FIG. 5 is EPMA images of a sintered body in a situation where x=0.50,y=0.30, x/y=1.67, and n=5.2 in the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO and shows the SEI image, BEI imageand X-ray image of Fe, respectively, from left to right on the upper rowand the X-ray images of La, Ca and Co, respectively, from left to righton the lower row.

FIG. 6 is EPMA images of a sintered body in a situation where x=0.50,y=0.20, x/y=2.50, and n=5.2 in the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO and shows the SEI image, BEI imageand X-ray image of Fe, respectively, from left to right on the upper rowand the X-ray images of La, Ca and Co, respectively, from left to righton the lower row.

FIG. 7 is EPMA images of a sintered body in a situation where x=y=0.5,x/y=1, and n=5.4 in the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO and shows the SEI image, BEI imageand X-ray image of Fe, respectively, from left to right on the upper rowand the X-ray images of La, Ca and Co, respectively, from left to righton the lower row.

FIG. 8 is graphs showing how the remanence Br, coercivity HcJ and loopsquareness Hk/HcJ of a sintered magnet change with the mole fraction ofH₃BO₃ added.

FIG. 9 is graphs showing how the remanence Br, coercivity HcJ and loopsquareness Hk/HcJ of a sintered magnet change with the mole fractions ofCaO and SiO₂ added.

FIG. 10 is graphs showing how the remanence Br, coercivity HcJ and loopsquareness Hk/HcJ of a sintered magnet change with the mole fraction y′of NiO.

BEST MODE FOR CARRYING OUT THE INVENTION

An oxide magnetic material according to the present invention isrepresented by the general formula:

(1−x)CaO.(x/2)R₂O₃.(n−y/2)Fe₂O₃ .yMO,

The present inventors paid special attention to the fact that the CaLaCoferrite has higher anisotropic magnetic field H_(A) than the SrLaCoferrite and carried out extensive researches on how to improve theperformance of the CaLaCo ferrite. As a result, the present inventorsdiscovered that the CaLaCo ferrite represented by the above formula hadbest ranges for its R mole fraction x, its M mole fraction y and its nvalue and that an oxide magnetic material having high Br, high HcJ andgood loop squareness could be obtained by getting R and M included suchthat x and y satisfied a particular ratio. R and M will be described indetail later.

A CaLaCo ferrite is disclosed in Patent Document No. 3. The preferredrange of x and y should be from 0.4 to 0.6 according to the descriptionof its examples. As for the x/y ratio, basically, x=y (i.e., x/y=1) issupposed to be satisfied. And only examples in which x/y=1.05 andx/y=1.16 are shown. It should be noted that in Patent Document No. 3,the mole fraction of Fe and Co in the general formula representing itscomposition is represented by z and there is no description about the nvalue.

As described above, the CaLaCo ferrite of Patent Document No. 3 has highBr and high HcJ but has poor loop squareness Hk/HcJ. This is probablybecause if the CaLaCo ferrite satisfies x=y=0.4 to 0.6, then heterophases including a lot of Co will be produced in the crystal structure,thus causing the decrease in loop squareness.

The present inventors looked for a composition that would not producesuch hetero phases. As a result, the present inventors discovered thatby adopting an x range of 0.4 to 0.6, a lower y range of 0.2 to 0.35,and an x/y ratio of 1.4 to 2.5, a material with high Br and high HcJcould be obtained. In a preferred embodiment of the present invention,magnetic properties including an HcJ of 370 kA/m or more and a Br of0.45 T or more, both of which are superior to the best propertiesdisclosed in Patent Document No. 3, are achieved. According to thepresent invention, a loop squareness of 85% or more is achieved in abroad range in which 4≦n≦6 was satisfied, a loop squareness of 90% ormore is achieved in a range in which 4.8≦n≦5.8 was satisfied, and thematerial with the high Br, high HcJ and a loop squareness of 90% or morewas obtained in a range in which 5.0≦n≦5.4 was satisfied.

The present invention is an improvement of a CaLaCo ferrite and includesCa as an essential element. According to the present invention, only Cais used instead of Sr and Ba.

R is at least one element selected from the group consisting of La, Ndand Pr and always includes La. Other elements, of which the ionic radiiare close to Sr²⁺ (e.g., Ce, Sm, Eu and Gd), may be included.

M is at least one element selected from the group consisting of Co, Zn,Ni and Mn and always includes Co. Other elements may be included as longas they are inevitably contained impurities.

According to the present invention, Co may be partially replaced withZn, Ni or Mn as described above. Any of Zn, Ni and Mn may be adopted asthe substituent because Br and HcJ that are higher than those of theSrLaCo ferrite disclosed in Patent Documents Nos. 1 and 2 are achievedin any case. Particularly, by replacing a portion of Co with Ni themanufacturing cost can be reduced without deteriorating the magneticproperties. Also, if Co is partially replaced with Zn, HcJ will decreaseto a certain degree but Br can be increased. The mole fraction of thesubstituent Zn, Ni or Mn is at most 50% of Co.

The mole fraction x shows the content of R and preferably satisfies0.4≦x≦0.6. This is because if x were less than 0.4 or more than 0.6, Brand the loop squareness would decrease.

The mole fraction y shows the content of M and preferably satisfies0.2≦y≦0.35. As described above, a preferred y range for the CaLaCoferrite was believed to be from 0.4 to 0.6 in the prior art. In thatcase, however, hetero phases including a lot of Co would be produced inthe crystal structure. The present invention is characterized byadopting a y range of 0.2 to 0.35 and by setting a particular x/y ratioas will be described later. If y were less than 0.2, Br and HcJ wouldboth decrease. However, if y were more than 0.35, then hetero phasesincluding a lot of Co would be produced and HcJ would decrease, which isnot beneficial.

The n value defining the ratio of CaO and R₂O₃ to Fe₂O₃ and MOpreferably satisfies 4≦n≦6, in which a loop squareness Hk/HcJ of 85% ormore is achieved. More preferably, n satisfies 4.8≦n≦5.8, in which aloop squareness of 90% or more is achieved. By setting the n value inthis range and adopting the preferred x and y ranges described above,properties including a Br of 0.45 T or more and an HcJ of 370 kA/m(=4.65 kOe) or more are achieved. Also, in the most preferred range, notonly the properties described above but also a loop squareness of 95% ormore are achieved. It should be noted that since it is difficult tomeasure the loop squareness of a calcined body, the loop squareness of asintered magnet is measured instead.

Hereinafter, a method of making an oxide magnetic material according tothe present invention will be described.

First, material powders of CaCO₃, Fe₂O₃, La₂O₃, CO₃O₄ and so on areprepared. The prepared powders are combined together such that x, y andn fall within their preferred ranges according to the general formuladescribed above. The material powders may include not just oxides andcarbonates but also hydroxides, nitrates and chlorides and may be in theform of solution as well. Also, in producing a sintered magnet, thematerial powders other than CaCO₃, Fe₂O₃ and La₂O₃ may be added eitherwhen the powders are mixed or after the calcining process (to bedescribed later) is finished. For example, after CaCO₃, Fe₂O₃ and La₂O₃have been combined, mixed together and calcined, CO₃O₄ and so on may beadded thereto and the mixture may be pulverized, compacted and thensintered. Optionally, to promote the reactivity during the calciningprocess, approximately 1 mass % of a compound including B₂O₃ and H₃BO₃may be added.

Among other things, the addition of H₃BO₃ is particularly effective inincreasing HcJ and Br. H₃BO₃ is preferably added up to 0.2 mass %, mostpreferably in the vicinity of 0.1 mass. In that case, as long as the nvalue and the mole fractions x and y fall within their preferred rangesdescribed above, properties including a Br of 0.45 T or more and an HcJof 370 kA/m or more are realized. If less than 0.1 mass % of H₃BO₃ isadded, then Br will increase significantly. On the other hand, if morethan 0.1 mass % of H₃BO₃ is added, then HcJ will increase noticeably.However, if more than 0.2 mass % of H₃BO₃ was added, then Br woulddecrease, which is not beneficial. That is why when used in applicationsin which Br plays a key role, 0.05 mass % to 0.15 mass % of H₃BO₃ ispreferably added. Meanwhile, when used in applications in which HcJplays an important role, 0.10 mass % to 0.20 mass % of H₃BO₃ ispreferably added. H₃BO₃ also has the effect of controlling crystalgrains during a sintering process. For that reason, it is also effectiveto add H₃BO₃ after the calcining process (i.e., before the finepulverization process or before the sintering process). Thus, H₃BO₃ maybe added both before and after the calcining process.

The material powders may be combined together by either a wet process ora dry process. When stirred up with a medium such as steel balls, thematerial powders can be mixed more uniformly. In a wet process, water isused as the solvent. Optionally, a known dispersant such as ammoniumpolycarboxylate or calcium gluconate may be used in order to dispersethe material powders. The mixed material slurry is dehydrated to be amixed material powder.

Next, the material powder mixture is heated by using an electric furnaceor a gas furnace, for example, thereby producing a ferrite compoundhaving a magnetoplumbite structure through a solid-phase reaction. Thisprocess will be referred to herein as “calcining” and a compoundobtained by this process will be, referred to herein as a “calcinedbody”.

The calcining process is preferably carried out in an atmosphere with anoxygen concentration of 5% or more. This is because the solid-phasereaction would not advance smoothly in an atmosphere that has an oxygenconcentration of less than 5%. More preferably, the oxygen concentrationis 20% or more.

In the calcining process, as the temperature rises, a ferrite phase isgradually formed through the solid-phase reaction. The formation of theferrite phase is completed at about 1,100° C. If the calcining processwere finished at a temperature lower than about 1,100° C., thenunreacted hematite would remain to deteriorate the resultant magnetproperties. The effects of the present invention are achieved if thecalcining temperature exceeds 1,100° C. However, if the calciningtemperature exceeded 1,450° C., then various inconveniences might becreated. For example, crystal grains might grow so much that it wouldtake a lot of time to pulverize the powder in the subsequent pulverizingprocess step. In view of these considerations, the calcining temperatureis preferably in the range of 1,100° C. to 1,450° C., more preferably1,200° C. to 1,350° C. Also, the calcining process is preferably carriedout for 0.5 to 5 hours.

If H₃BO₃ has been added before the calcining process, then the abovereaction will be promoted so much that the calcining process can becarried out at a temperature of 1,100° C. to 1,300° C.

The calcined body obtained by this calcining process has a main phase ofa ferrite having a hexagonal M-type magnetoplumbite structurerepresented by the chemical formula:

(1−x)CaO.(x/2)R₂O₃.(n−y/2)Fe₂O₃ .yMO,

where 0.4≦x≦0.6, 0.2≦y≦0.35 and 4≦n≦6, and becomes the oxide magneticmaterial of the present invention.

By pulverizing and/or crushing such a calcined body, a magnetic powdercan be obtained and can be used to make a bonded magnet or a magneticrecording medium. Optionally, the calcined body may be made by a knownmanufacturing technique such as a spray pyrolysis process or acoprecipitation process.

When used to make a bonded magnet, the magnetic powder is mixed with arubber with some flexibility or a hard and lightweight plastic. Then,the mixture is subjected to a compaction process, which may be carriedout by a method such as injection molding, extrusion molding or rollmolding. Also, when applied to a bonded magnet, the magnetic powder ispreferably thermally treated at a temperature of 700° C. to 1,100° C.for about 0.1 to about 3 hours in order to relax the crystal strain ofthe magnetic powder. A more preferred temperature range is from 900° C.to 1,000° C.

Meanwhile, when used to make a magnetic recording medium, the magneticpowder may be subjected to the heat treatment process described above,mulled with any of various known binders, and then the mixture isapplied onto substrate. In this manner, a coated magnetic recordingmedium can be obtained. Alternatively, a thin-film magnetic layer foruse in a magnetic recording medium may be formed by a sputteringprocess, for example, with the oxide magnetic material of the presentinvention and a sintered magnet including the material used as a target.

Next, a method for producing a sintered magnet using this oxide magneticmaterial will be described.

The calcined body is subjected to a fine pulverization process using avibrating mill, a ball mill and/or an attritor so as to be pulverizedinto fine powder particles having a mean particle size of about 0.4 μmto about 0.8 μm as measured by the air permeability method. The finepulverization process may be either dry pulverization or wetpulverization but is preferably carried out as combination of these twotypes of pulverization processes.

The wet pulverization process may be carried out using an aqueoussolvent such as water or any of various non-aqueous solvents includingorganic solvents such as acetone, ethanol and xylene. As a result of thewet pulverization process, slurry is produced as a mixture of thesolvent and the calcined body. Any of various known dispersants orsurfactants is preferably added to the slurry at a solid matter ratio of0.2 mass % to 2.0 mass %. After the wet pulverization process is over,the slurry is preferably condensed and mulled.

In the fine pulverization process, CaCO₃, SiO₂, Cr₂O₃ or Al₂O₃ may beadded to the calcined body to improve the magnetic properties thereof.If any of these additives is added, 0.3 mass % to 1.5 mass % of CaCO₃when converted into the mass of CaO, 0.2 mass % to 1.0 mass % of SiO₂,at most 5.0 mass % of Cr₂O₃ or at most 5.0 mass % of Al₂O₃ is preferablyadded when converted into CaO.

Among other things, it is particularly effective to add CaCO₃ and/orSiO₂. And if CaCO₃ and/or SiO₂ and H₃BO₃ are added in combination, highBr and high HcJ are achieved. SiO₂ also has the effect of controllingcrystal grains during the calcining process. For that reason, it is alsoeffective to add SiO₂ before the calcining process. Thus, SiO₂ may beadded both before the calcining process and before the finepulverization process.

Thereafter, the slurry is pressed and compacted with or without amagnetic field applied thereto, while the solvent is removed from theslurry. By pressing and compacting the slurry under a magnetic field,the crystallographic orientations of the powder particles can be alignedwith each other. As a result of the compaction process under themagnetic field, the magnetic properties can be improved significantly.Optionally, to further align the orientations, 0.01 mass % to 1.0 mass %of dispersant or lubricant may be added.

The compacts formed by the compaction process are subjected to adegreasing process, if necessary, and then to a sintering process, whichmay be carried out using an electric furnace or a gas furnace, forexample.

The sintering process is preferably carried out in an atmosphere thathas an oxygen concentration of at least 10%. If the oxygen concentrationwere lower than 10%, then excessive particle growth would be causedand/or hereto phases would be produced, thus possibly deteriorating themagnetic properties, which is a problem. The oxygen concentration ismore preferably 20% or more, and most preferably 100%.

As will be described later by way of specific examples, even when bakedin the air, the oxide magnetic material of the present inventionexhibits magnetic properties that are at least as good as those realizedby baking the CaLaCo ferrite in oxygen (i.e., in 100% oxygen) asdisclosed in Patent Document No. 3. That is why by baking the oxidemagnetic material of the present invention in oxygen just as disclosedin Patent Document No. 3, even better magnetic properties are realized.

The sintering process is preferably carried out at a temperature of1,150° C. to 1,250° C. for 0.5 to 2 hours. A sintered magnet obtained bythe sintering process has an average crystal grain size of approximately0.5 μm to approximately 2 μm.

After having been sintered, the sintered compact is subjected to variousknown manufacturing processing steps including finishing, cleaning andtesting to complete a ferrite sintered magnet as a final product.

EXAMPLES Example 1

First, a CaCO₃ powder, an La₂O₃ powder, an Fe₂O₃ powder (with a particlesize of 0.6 μm) and CO₃O₄, were prepared and mixed together such that acomposition (1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO would satisfy x=0.5,1≦x/y, 0≦y≦0.5 and n=5.4. The resultant material powders were mixedtogether in a wet ball mill for four hours, dried, and then sieved.Thereafter, the powder was calcined in the air at 1,300° C. for threehours, thereby obtaining a calcined body in the form of powder.

Next, 0.6 mass % of CaCO₃ powder (when converted into the mass of CaO)and 0.45 mass % of SiO₂ powder were further added to the calcined body.Then, using water as a solvent, the mixture was finely pulverized in awet ball mill to a mean particle size of 0.55 μm as measured by the airpermeability method. Thereafter, while the solvent was removed from theresultant finely pulverized slurry, the slurry was pressed and compactedunder a magnetic field. The compaction process was performed such thatthe pressing direction became parallel to the direction of the magneticfield, which had a strength of 13 kOe. Next, the resultant compact wassintered in the air at 1,150° C. for one hour to make a sintered magnet.

The magnetic properties of the sintered magnet thus produced weremeasured. When the abscissa represents the x/y ratio of the molefraction x of La to the mole fraction y of Co, the remanence Br,coercivity HcJ and loop squareness Hk/HcJ measured change as shown inFIG. 1. On the other hand, when the abscissa represents the molefraction of y added, Br, HcJ and Hk/HcJ change as shown in FIG. 2.

As is clear from FIG. 1, if the x/y ratio was too low, HcJ and Hk/HcJdecreased due to the presence of hetero phases. When x/y wasapproximately 1.25 or less, HcJ was smaller than 340 kA/m (=4.27 kOe).And when x/y was 1.4 or less, Hk/HcJ was lower than 85%. On the otherhand, if the x/y ratio was too high, Br and HcJ decreased. For example,when x/y was approximately 2.5 or more, Br was smaller than 0.44 T andHcJ was smaller than 340 kA/m (=4.27 kOe). In the prior art, the bestx/y ratio was believed to be approximately equal to one considering thecharge correction relationship. However, it can be seen that the CaLaCoferrite of the present invention realized high magnetic properties whensatisfying 1.4≦x/y≦2.5.

Also, as is clear from FIG. 2, if the mole fraction of the substituentCo was too low, Br and HcJ decreased. For example, when y wasapproximately 0.2 or less, Br was smaller than 0.44 T and HcJ wassmaller than 340 kA/m (=4.27 kOe). On the other hand, if the molefraction of y was too high, HcJ and Hk/HcJ decreased due to the presenceof hetero phases. When y was approximately 0.4 or more, HcJ was smallerthan 340 kA/m (=4.27 kOe). And when y was 0.35 or more, Hk/HcJ was lowerthan 85%. It can be seen that high magnetic properties were realizedaccording to the present invention when 0.20≦y≦0.35 was satisfied.

Example 2

A sintered magnet was produced as in the first specific exampledescribed above except that the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO satisfied x=0.5, y=0.3, x/y=1.67,and 3.6≦n≦6.0. The magnetic properties of the sintered magnet thusproduced were measured. When the abscissa represents the n value, theremanence Br, coercivity HcJ and loop squareness Hk/HcJ measured changeas shown in FIG. 3.

As is clear from FIG. 3, a good loop squareness of 85% or more wasachieved in the range in which 4.0≦n≦6.0 was satisfied and a better loopsquareness of 90% or more was achieved in the range in which 4.8≦n≦5.8was satisfied. Furthermore, when 5.0≦n≦5.4 was satisfied, good magneticproperties including a Br of 0.44 T and an HcJ of 340 kA/m (=4.27 kOe)were realized. And when n=5.2, an HcJ of 370 kA/m or more and a Br of0.45 or more were achieved.

Example 3

In the composition of the second specific example, the magneticproperties were measured with n fixed at 5.2 and with the sinteringtemperature changed within the range of 1,150° C. to 1,190° C. Theresults are shown in the following Table 1:

TABLE 1 Sintering temperature Br HcJ (BH) max Jr/Js Hk/HcJ (° C.) (T)(kA/m) (kJ/m³) (%) (%) 1150 0.453 370.0 40.5 98.9 97 1170 0.460 300.241.3 98.9 96 1190 0.464 271.2 42.0 99.1 92

As can be seen from Table 1, when the sintering temperature wasrelatively low, a high HcJ of 370 kA/m (=4.65 kOe) and a high Hk/HcJratio exceeding 95% were achieved. On the other hand, when the sinteringtemperature was relatively high, a high Br exceeding 0.46 T and a highHk/HcJ ratio exceeding 90% were achieved.

Example 4

A sintered magnet was produced as in the first specific exampledescribed above except that the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO satisfied 0≦x≦1, y=0.3 and n=5.2.The magnetic properties of the sintered magnet thus produced weremeasured. When the abscissa represents the mole fraction of x added, theremanence Br, coercivity HcJ and loop squareness Hk/HcJ measured changeas shown in FIG. 4.

As can be seen from FIG. 4, if the mole fraction x is too low or toohigh, Br and Hk/HcJ decrease significantly. It can also be seen that ahigh Hk/HcJ ratio of 95%, high Br and high HcJ are achieved when0.4≦x≦0.6 was substantially satisfied.

Example 5

A sintered magnet was produced as in the first specific exampledescribed above except that the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO satisfied x=0.5, y=0.3 or 0.2,x/y=1.67 or 2.5 and n=5.2. The sintered magnet obtained was subjected toa composition analysis using an EPMA. The results of this analysis areshown in FIG. 5 (in which x/y=1.67) and in FIG. 6 (in which x/y=2.5).The EPMA analysis was carried out using an electron probe microanalyzerEPMA 1610 (produced by Shimadzu Corp.) under the conditions including anaccelerating voltage of 15 kV, a sample current of 0.1 μA, and anirradiation range φ of 100 μm (as represented by the electron beamdiameter).

As can be seen from FIGS. 5 and 6, no hetero phases including a lot ofCo were identified in the sintered magnet of the present invention.Consequently, excellent magnetic properties were realized as in thefirst through fourth specific examples described above.

Comparative Example 1

A sintered magnet was produced as in the first specific exampledescribed above except that the composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO satisfied x=0.5, y=0.5, x/y=1 andn=5.4. The sintered magnet obtained was subjected to a compositionanalysis using an EPMA. The results of this analysis are shown in FIG.7. The EPMA analysis was carried out under the same conditions as thoseadopted in the fifth specific example.

As is clear from FIG. 7, a great number of hetero phases including a lotof Co (i.e., white spots on the photo at the lower right corner of FIG.7) were identified in the sintered magnet of the comparative example.The magnetic properties of the sintered magnet were measured. As aresult, Br was 0.441 T, HcJ was 325.5 kA/m (=4.09 kOe) and Hk/HcJ was63%. Among other things, the Hk/HcJ ratio decreased significantly, whichshould be because of the presence of those hetero phases including a lotof Co.

Example 6

First, a CaCO₃ powder, an La₂O₃ powder, an Fe₂O₃ powder (with a particlesize of 0.6 μm) and a CO₃O₄ powder were prepared such that a composition(1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCaO would satisfy x=0.5, y=0.3,x/y=1.67, and n=5.2. These material powders were combined together and 0to 0.2 mass % of H₃BO₃ powder was further added thereto. The resultantmaterial powders were mixed together in a wet ball mill for four hours,dried, and then sieved. Thereafter, the powder was calcined in the airat 1,150° C. for three hours, thereby obtaining a calcined body in theform of powder.

Next, 0.6 mass % of CaCO₃ powder (when converted into the mass of CaO)and 0.45 mass % of SiO₂ powder were further added to the calcined body.Then, using water as a solvent, the mixture was finely pulverized in awet ball mill to a mean particle size of 0.55 μm as measured by the airpermeability method. Thereafter, while the solvent was removed from theresultant finely pulverized slurry, the slurry was pressed and compactedunder a magnetic field. The compaction process was performed such thatthe pressing direction became parallel to the direction of the magneticfield, which had a strength of 13 kOe. Next, the resultant compact wassintered in the air at 1,200° C., for one hour to make a sinteredmagnet.

The magnetic properties of the sintered magnet thus produced weremeasured. When the abscissa represents the amount of H₃BO₃ added, theremanence Br, coercivity HcJ and loop squareness Hk/HcJ measured changeas shown in FIG. 8.

As can be seen easily from FIG. 8, when 0.1 mass % of H₃BO₃ was added,Br and HcJ were both good. However, if H₃BO₃ was smaller than 0.1 mass%, Br increased but HcJ decreased. Conversely, if H₃BO₃ was greater than0.1 mass %, HcJ increased but Br decreased. The loop squareness was 85%or more.

Example 7

A sintered magnet was produced as in the sixth specific example exceptthat 0.1 mass % of H₃BO₃ powder, 0.5 mass % to 0.9 mass % of CaCO₃powder (when converted into the mass of CaO) and 0.3 mass % to 0.9 mass% of SiO₂ powder were added. The magnetic properties of the sinteredmagnet thus produced were measured. When the abscissa represents theamount of SiO₂ added, the remanence Br, coercivity HcJ and loopsquareness Hk/HcJ measured change as shown in FIG. 9, in which the solidcircles represent a situation where 0.5 mass % of CaO was added, thesolid triangles represent a situation where 0.7 mass % of CaO was added,and the solid squares represent a situation where 0.9 mass % of CaO wasadded.

As can be seen from FIG. 9, the CaLaCo ferrite of the present inventionexhibited excellent properties when approximately 0.7 mass % of CaCO₃(when converted into the mass of CaO) and approximately 0.6 mass % ofSiO₂ were added.

Example 8

A sintered magnet was produced as in the first specific example exceptthat 0.1 mass % of H₃BO₃ powder, 0.7 mass % of CaCO₃ powder (whenconverted into the mass of CaO) and 0.6 mass % of SiO₂ powder wereadded, the calcining temperature was 1,225° C. and the sinteringtemperature was either 1,190° C. or 1,200° C. The magnetic properties ofthe sintered magnet thus produced were measured. The results are shownin the following Table 2:

TABLE 2 Sintering temperature Br Hcj (BH) max Hk/HcJ (° C.) (T) (kA/m)(kJ/m³) (%) 1190 0.449 436.4 39.4 92 1200 0.454 412.4 40.1 89

It can be seen that by adopting preferred calcining and sinteringtemperatures in combination with the preferred amounts of CaCO₃ and SiO₂added in this seventh specific example, even better Br and HcJ wereachieved.

Example 9

First, a CaCO₃ powder, an La₂O₃ powder, an Fe₂O₃ powder (with a particlesize of 0.6 μm), an NiO powder and a Co₃O₄ powder were prepared suchthat a composition (1−x)CaO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCaO.y.NiO wouldsatisfy x=0.5, y+y′=0.3, y′=0 to 0.1, x/y=1.67, and n=5.2. Thesematerial powders were combined together and 0.1 mass % of H₃BO₃ powderwas further added thereto. The resultant material powders were mixedtogether in a wet ball mill for four hours, dried, and then sieved.Thereafter, the powder was calcined in the air at 1,150° C. for threehours, thereby obtaining a calcined body in the form of powder.

Next, 0.7 mass % of CaCO₃ powder (when converted into the mass of CaO)and 0.6 mass % of SiO₂ powder were further added to the calcined body.Then, using water as a solvent, the mixture was finely pulverized in awet ball mill to a mean particle size of 0.55 μM as measured by the airpermeability method. Thereafter, while the solvent was removed from theresultant finely pulverized slurry, the slurry was pressed and compactedunder a magnetic field. The compaction process was performed such thatthe pressing direction became parallel to the direction of the magneticfield, which had a strength of 13 kOe. Next, the resultant compact wassintered in the air at 1,190° C. for one hour to make a sintered magnet.

The magnetic properties of the sintered magnet thus produced weremeasured. When the abscissa represents the mole fraction of y′ (i.e.,NiO added), the remanence Br, coercivity HcJ and loop squareness Hk/HcJmeasured change as shown in FIG. 10.

As can be seen easily from FIG. 10, even if Co was replaced with Ni,neither Br nor HcJ decreased significantly. Ni is less expensive thanCo. Thus, by replacing Co with Ni, the manufacturing cost can be reducedwithout deteriorating the magnetic properties.

INDUSTRIAL APPLICABILITY

The oxide magnetic material of the present invention has high remanenceBr, high coercivity HcJ and good loop squareness, and therefore, can beused effectively to make high-performance motors, for example.

1. A method of producing an oxide magnetic material having a composition represented by the formula: (1−x)CaO.(x/2)R₂O₃.(n−y/2)Fe₂O₃ .yMO, where R is at least one element selected from the group consisting of La, Nd and Pr and always includes La, M is at least one element selected from the group consisting of Co, Zn, Ni and Mn and always includes Co, and the mole fractions x, y and n satisfy 0.4≦x≦0.6, 0.2≦y≦0.35, 4≦n≦6, and 1.4≦x/y≦2.5, wherein the oxide magnetic material consisting essentially of a ferrite having a hexagonal M-type magnetoplumbite structure, the method comprises the steps of: preparing a mixed powder to realize a composition represented by the formula; calcining the mixed powders to form the oxide magnetic material having the composition represented by the formula; and adding at most 0.2 mass % of H₃BO₃ before and/or after the step of calcining.
 2. The method of claim 1, wherein 4.8≦n≦5.8 is satisfied. 